Blue naevi and the blue tumour spectrum

Blue naevi (BN) form a wide group of benign dermal melanocytic proliferations. They are genetically distinct from common and Spitz naevi with frequent hotspot mu-tations occurring in Gaq genes.Clinically, BN display a female predominance, elective sites of emergence and a great variety of subtypes related to specific regions of the skin linked to early embryological genetic events. Histologically, most BN are located in the dermis with small, bland, spindled and dendritic pigmented melanocytes within a fibrous background. Variation in tumour volume, fibrosis, and melanin pigment load can be broad. A growth in size and cellularity can occur within a subset of tumours as they acquire the morphological fea-tures of cellular blue naevi, with a biphasic architecture associating a dendritic blue naevus morphology near the surface, and deep vertical cellular expansions of medium-sized, bland melanocytes often reaching the subcutis. Sclerosing and myxoid variants can be observed either as individual or combined modifications that can add complexity to an otherwise straightforward diagnosis. Ma-lignant progression of a cellular blue naevus is exceptional with an intermediate stage named atypical cellular blue naevus. Malignant blue melanomas are fast growing, large, pigmented tumours with most often obvious features of malignancy. However, they are difficult to separate from other malignant dermal melanocytic proliferations.Herein, we will extensively detail and illustrate the clinical, histological and genetic features of the vast spectrum of blue naevi and related entities in the skin. an underlying neural distribution. Similarly to congenital naevi, lesions from the blue group are for the most part linked to an accidental genetic event in the neural crest during embryogenesis with a migration towards the skin of mela-noblasts bearing specific genetic anomalies. The clinical localisations in which most blue naevi are found are the distal dead-end zone of the migration path of the melanoblasts. These predominant areas are the distal part of extremities, especially the back of the hand or foot, parieto-temporal ce-phalic area and the gluteal area (related to the vestigial tail). This review will only cover blue tumours located in the skin. Table 1 links all the terminologies used in this review with the matching histological grade.